A concept in game theory conceived of by mathematician John Nash.
A Nash equilibrium is the best choice of strategy in a game, regardless of the strategies of opposing parties.
An example of a Nash equilibrium can be found in the game known as the Prisoner's Dilemma, where it is better for each prisoner to choose to betray his/her partner in crime, rather than cooperate with the authorities and risk being betrayed (though the same does not apply to iterated versions of this game).